Uranus: The Seventh Planet

This poor planet suffers that name that sounds like two words your Proctologist might put together when discussing your condition. It is laughed at so often that it inspired an article by a man name DeCotis. I cannot locate the original article but I emailed him the following message. I hasten to point out that this text – once sent – has been augmented, improved, embellished and even illustrated over the years and especially just prior to the posting of this article.

Mr. DeCotis,

Heartiest congrats to Space Online, Billy Cox and yourself on a wonderful bit of writing about the planet Uranus. I myself have long considered the name of first trans-Saturnian planet to be a problem. This became a matter of importance when, in 1977, it was discovered that, like other Gas Giants, “Neptune-Minus-One” had rings. This was before Voyager 2 got to #7 and was accomplished by watching that planet pass in front of a star. This is called an occultation. Unexpectedly, the star dimmed several times before and after the planet covered it. Only rings could explain it since expecting that many satellites to be lined up in that fashion was improbable in the extreme.

Knowing me to be an Astronomy student, people would ask me, “Are there really rings around Uranus?”. I understood that as a very personal and offending question and I was tempted to demonstrate the (negative) answer visually, but I refrained.

Actually, I explained to them about the occultation, just as in the first paragraph – being a thoroughgoing Astronomy nerd.

There was a Science Fiction B-movie about Uranus which was euphemistically entitled “Journey to the Seventh Planet” back in the sixties*. Even as a teenager (or especially as a teenager, I suppose) it didn’t take long to figure out what they were avoiding. There was a brief movement (no disgusting pun intended) to transfer the emphasis to the first syllable but you can see (well, hear) immediately that this is a non-starter (“Urine-us”). The name would still be in the bathroom humor department and would only prompt a new round of adolescent jokes.

It was about then that I decided that “Joe” was a nice enough name. But in order to differentiate whether we were talking about Lewis, Dimaggio, Cool, College, Blow, Six Pack or the Planet, we’d need to specify “Joe the Planet” for every reference. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Joe the Planet, Neptune and Pluto. I’ll grant you it’s a bit awkward at first but it should put an end to the pubescent snickering in astronomy lectures and planetarium shows.Sincerely,

Steve

*I looked it up on YouTube recently – it still stinks.

In college, I was assured by actual Astronomy Professors that this planet’s name is “your ah noose” (accent on “ah” and “noose” rhymes with moose). The other pronunciations you may have heard are either erroneous or may be attributed to the aforementioned “bathroom humor”.

Now that we have the nomenclature issue dealt with, let’s have a look at the planet itself.

Discovery:

Quoting a NASA planet resource website{1]:

“The first planet found with the aid of a telescope, Uranus was discovered in 1781 by astronomer William Herschel,although he originally thought it was either a comet or a star. It was two years later that the object was universally accepted as a new planet, in part because of observations by astronomer Johann Elert Bode.William Herschel tried unsuccessfully to name his discovery Georgium Sidus after King George III. Instead the planet was named for Uranus, the Greek god of the sky, as suggested by Johann Bode.”

So, it’s “Bode’s ill” – so to speak. Don’t blame poor Herschel for the double entendre. Nor his sister Caroline who joined the musician turned astronomer and accomplished many discoveries of her own:

“Caroline assisted Herschel until his death. She discovered eight comets. She also discovered several deep-sky objects and was the first woman to be given a paid scientific position…”

Early Observation

There was not much to see. Even in the most powerful “backyard” telescopes – as late as the 1980’s Uranus was a small dim pale blueish green dot. A “professional” telescope of that era would be required to resolve the largest satellite, Titania as a featureless point of light. Even in those elaborate instruments, Uranus maintained its elusive nature.

“Even through large telescopes the planet often appears fuzzy and indistinct. Brightness variations are sometimes reported, the likely result of changes in the planet’s atmosphere.” [2]

Figure 2: Uranus through a large “backyard” telescope.

Below is that table of planetary statistics that readers may have seen before.

Table 1: Statistics for the Planets

The seventh planet is 19 times the Earth’s distance from the Sun.

Figure 3: Simple “Visual aid” to depict the distance of Uranus (big green dot) from the Sun (Yellow asterisk) as compared to that of the Earth (little blue dot). Only the distances are to scale – not the sizes of the Sun and planets.

In size, it is 31, 763 miles in diameter (four and a half times that of Earth). Like all the Giant Planets, it rotates quickly (once every 17 hours and 12 minutes) and it is much less dense than the “Rocky Planets” like Earth.

The atmosphere is hydrogen and helium with some methane. Deeper down, there is a “mantle” of water, ammonia and methane ices above a rocky core. You see in figure 3A that they have not labeled the thicknesses of these layers. That is a sure sign that they don’t really have a clue what those numbers should be!

Figure 3A: Internal Structure of Uranus

A notable unique feature of Uranus is the orientation of its spin axis relative to the plane of its orbit (see “obliquity in orbit” in the table). In the Uranian summer and winter the axis of rotation of the planet points almost directly at the Sun – resulting in one hemisphere in constant sunlight and the other in darkness. This is thought to have been caused by Uranus’ collision with a large planetoid late in its formation. The diagram below explains the situation.

Figure 4: Seasons of Uranus

Uranus was visited by a space probe only once. It was the third stop on what was called at the time “The Grand Tour”. As it happened, there was an alignment of the outer planets in the 70’s and 80’s such that it would be possible to use gravity assisted orbital adjustments (“the slingshot effect”) to make it possible for a space probe to visit Jupiter, Saturn, Uranus and Neptune in one long and carefully managed trajectory. The Voyager 2 Spacecraft did exactly that and arrived in the area of Uranus in 1986.

The Voyager 2 Spacecraft

The Voyager probes each had a main antenna that was capable of constant communications with the Earth. This necessitated what is called a “scan platform” that held the instruments that need precise pointing and moved independently of the antenna. The constant contact was needed because data storage was actually on a ½ inch, 8 track magnetic tape with a total capacity of about ½ Megabyte and a top baud rate of 56kilobits per second (2). That’s what I said – “Stone Knives and Bear Skins!” – so, real-time transmission was required for image data.” Voyager was – despite my demeaning reference – quite advanced at the time and its imagery and other data are still quite impressive. They made the most of the technology at hand.

The image below depicts the identical Voyager 1 and Voyager 2 Spacecraft. (4) The dish antenna is 3.7 meters in diameter (12 feet, 2 inches) across. The arm extending to the right contains the main experiments and the imaging “scan platform”. The left arm holds the three radioisotope thermoelectric generators that provided the electric power out in the dark reaches where solar panels would be quite ineffective. The gold disk on the “body” is the famous Record with messages and images of Earth for anyone “out there”. Carl Sagan, whose enthusiasm for the search for extraterrestrial intelligence (SETI) was well-known had thought to perhaps include a plaque with a message engraved upon it as had been done with the Pioneer space probes. This Record (an actual grooved phonograph Long Playing (LP) disc – only metal, not vinyl) was the idea of Frank Drake. SETI Nerds will recognize Drake as the inventor of the “Drake Equation” which is a formula to calculate how many extraterrestrial civilizations there might be. That’s Frank in the inset, with his equation. I put him there to give scale to the picture.

Figure 5: The Voyager Spacecraft NASA/NASA website

Figure 6: A Voyager view of Uranus in 1986.

Even the dedicated planetary scientists had to admit they were disappointed with the rather very bland appearance of the planet. In trying to describe the feelings of the Voyager team about the mediocrity of it all, Planetary Scientist Heidi Hammel had this to say, “…poor Uranus…poor Uranus!”.[6]

There had been observations from Earth of clouds in this atmosphere, so what’s the deal? You will see in the diagram in figure 4 that the solstice – that point in the orbit where one hemisphere is constantly roasting in sunlight – was in 1986. Just when Voyager happened along. Later observations were made with (much improved) telescopes in the years surrounding the Equinox of 2007 (see Figure 4) – when most of Uranus has 8 ½ hours sun and 8 ½ hours darkness – “barbeque” mode, as they say. Those images showed Uranus in its more “flamboyant” mood. Figure 7.

Figure 7: Uranus near Equinox. Note the rings (R) – now markedly evident when they are seen edge-on.

It is probably worth noting that the Voyager camera and those of the Hubble Space Telescope (HST) are almost certainly quite different in their ranges of wavelengths and sensitivities, so they are not directly comparable. Nonetheless, we may expect more blandness from “poor Uranus” around the Summer Solstice in 2028.

I should mention that there is a lot more science involved than just the images recorded by Voyager and results from those found new and interesting features, as well. For example, the magnetic field detected is not centered on the planet core and its poles are near the rotational equator. This was totally unexpected.

The Satellite that “Saved the Show”

One of the major aspects of interest in the Giant planets was the characteristics and history of their satellites. With Jupiter and Saturn, the space probes entered and left the planetary systems obliquely across the orbits of the moons and could, with luck, come close to several of them for detailed examination. In the case of Uranus, the moons’ orbits are like circles on the sky and are approached as if in target practice. The “Grand Tour” scenario of hopping from one outer planet too the next required very specific trajectories past the planets along the way. That, and the angle of the sun left only one chance of a close approach to a satellite and even that would see only the perpetually lit hemisphere of the smallest of the major moons – Miranda (Figure 7) It could not have been predicted that this would be by far the most interesting of all the moons and the feature we could all point to when asked by non-Nerds why all this expense and effort was spent to go look at a blue-green billiard ball – with no number on it.

Figure 8: Miranda

Miranda is the smallest (about 300 miles across) of the major satellites and the closest to the planet (roughly 81,000 miles). It circles Uranus in 1.4 days and always shows the same face to the planet. This is looking down at the South pole. In the season when Voyager arrived, this was pretty much all that would have been illuminated.

And, it looks like it has been broken apart and then shoved back together! Not surprisingly, that is one idea of how it came to look so.

“Scientists disagree about what processes are responsible for Miranda’s features. One possibility is that the moon may have been smashed apart in some colossal collision, and the pieces then haphazardly reassembled. Another, perhaps more likely, scenario is that the coronae are sites of large rocky or metallic meteorite strikes which partially melted the icy subsurface and resulted in episodic periods of slushy water rising to Miranda’s surface and refreezing.”[8]

Uranus has four larger satellites. The biggest is Titania which is still less than half the diameter of the Earth’s moon. As mentioned earlier they were not well surveyed in the fly-by, but a map of Titania’s surface appears in figure 9.

Figure 9: A Map of Titania’s surface. Again, only about half the surface was illuminated and this is the least boring part of that.

The larger satellite also has some interesting surface features. I am reminded of my own varicose veins.

Conclusions

The seventh planet turns out to be rather dull and featureless, but only for the Southern Summer. The Spring Equinox brought considerable atmospheric activity after Voyager but now detectable from the improved cameras of the Hubble Space Telescope and other modern observatories.

The satellites of all the Giant Planets all turned out to be far more complex than was first imagined by Earthbound observers. Uranus is no exception.

Uranus still has that unfortunate name (despite my “Joe” recommendation) but we can overlook that because we are all adults, here. Right?…Right?

Update: In all that discussion of the unfortunate moniker, I neglected to mention a sports cheer, popular around the time of the ring discovery. It goes like this: “Up Jupiter! Up Saturn!…”
(So much for Adulthood 😉 )